With the 'collision' extension enabled, each time the user tries to create a new event or modify some existing one inside an already occupied time slot, the scheduler invokes
the onEventCollision event which checks the value set with the
collision_limit property.

But remember, the onEventCollision event isn't triggered while loading data. So, to control a
number of items in a time slot while data is being loaded to the scheduler, you need to extend the previous code a bit: